Next generation synthetic fibers UHPFRC for sustainable structural applications

The ever-increasing demand for rehabilitation of existing structures and construction of new ones, along with environmental issues of the construction sites represent a heavy burden for society in terms of economy and environment. Meanwhile, Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) has proved its potential to be one of the solutions to contain the explosion of the maintenance costs, because of its extremely low permeability and outstanding mechanical properties. Even though using UHPFRC already significantly contributes to decreasing the environmental impact of rehabilitation construction sites by up to 50% when compared to using conventional concretes, further optimization of their mix design with replacement of high environmental cost components like steel fibers and clinker, would be another step towards sustainability.

The main objective of this study was to further improve the already established concept of UHPFRC in two main directions: firstly, to develop new strain hardening UHPFRC mixes with reduced environmental impact by replacing the steel fibers with synthetic ones and by replacing 50% vol. clinker with cheap and widely available Supplementary Cementitious Materials, and secondly, to investigate the mechanical properties, delayed response, protective function, and environmental impact of the newly developed material for structural applications.

An advanced packing density model was developed to optimize the powder mix that consisted of six different powders. An extensive experimental campaign with a focus on the tensile behavior was carried out to investigate the factors influencing the mechanical properties of the UHPFRC mixes and validate their performances. The achieved new mix named PE-UHPFRC has on average a tensile elastic limit of 7.7 MPa, a tensile ultimate strength of 11.7 MPa, a tensile deformation capacity of more than 3.5%, and a compressive strength of 120 MPa, well adapted for cast-onsite applications of rehabilitation, or strengthening associated to rebar.

The delayed response of PE-UHPFRC was investigated with a focus on autogenous deformations and associated eigenstresses under full restraint conditions, using a Temperature Stress Testing Machine. The results showed more than 70% reduction in the eigenstresses in a 50 mm thick PE-UHPFRC layer when compared to those observed in conventional UHPFRC with steel fibers, which is highly beneficial for rehabilitation applications.

An original capillary absorption setup was developed to investigate the protective properties of the PE-UHPFRC under tensile loading, at different strain levels ranging from 0.15 to 20â°. Before reaching the tensile elastic limit, the capillary absorption coefficient was extremely low, at 24 g/m2âh, however, a sudden increase in the capillary absorption was observed after the elastic limit under tensile load.

Finally, the environmental benefits of PE-UHPFRC combined with rebar were characterized at the structural level, for the strengthening of a bridge with a span of 34 m, and compared with those of: (1) a reconstruction with prestressed concrete, and (2) a strengthening with conventional UHPFRC with steel fibers combined with rebar. The LCA results showed on average 55% and 34% decrease in the environmental impact for the strengthening method with R-PE-UHPFRC compared with the reconstruction with prestressed concrete and strengthening with conventional R-UHPFRC, respectively.

Time-Dependent Response of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) under Low to High Tensile Stresses

Agnieszka Ewa Switek

Ultra High Performance Fibre Reinforced Concretes (UHPFRC) are characterised by high mechanical performance, extremely low permeability and tensile strain hardening. For structural applications, in composite structures they are very well adapted for reinforcement and protection of existing concrete members. Cast in-situ, they undergo under restraint development of tensile eigenstresses at early age, which can lead in some specific cases of applications to localised macrocracking of a new layer and loss of protective function. On the other hand their tensile viscoelastic potential helps mitigate these stresses. The objective of this study was to investigate the tensile viscoelastic properties of UHPFRC at early ages and long-term in creep and relaxation tests. A large experimental campaign with tensile creep and relaxation tests was performed, with load levels ranging from 30 to 90% of the tensile strength, including creep and relaxation tests during the strain hardening phase, at two different loading ages of 3 and 7 days. Three main testing setups were used: TSTM (Temperature Stress Testing Machine), creep rigs and closed-loop servo-hydraulic testing machine with additional Acoustic Emission detection. The influence of early age evolution in mechanical response and underlying mechanisms governing this response were discussed. Linear viscoelastic models were used to analyse and to highlight on the basis of the experimental results the non-linear behavior depending on solicitation level and loading sequence. The non-linear threshold for viscoelastic response was estimated for the material loaded at 3 and 7 days. The early age mechanical response at 3 days was proportional to the load level in the instantaneous part for three stress levels tested 30, 60 and 90% of the tensile strength, and showed non-linear creep from 60% on. It was observed that for a very low stress level of 13% the creep deformation was decreasing. Relaxation tests with compensation of simultaneously monitored shrinkage deformation were performed, and showed a viscoelastic potential similar to the one observed for creep for the low load level of 30%. Incremental creep and relaxation tests were successfully performed for 3 days age specimens. For higher stress levels, the viscoelastic response from creep tests was more pronounced than in relaxation tests, which was highlighted by analysis with linear viscoelastic models. The creep response at 7 days showed that the threshold for non-linear viscoelasticity is close to 9 MPa (81% of the tensile strength) load level. The Acoustic Emission non-destructive testing method was used to get insight on the physical mechanisms involved in viscoelastic deformations. A strong correlation between AE number of events and the deformation was found, also the amplitude and energy of AE events was found to be of the same order during the instantaneous and delayed deformations. UHPFRC loaded at 7 days age in incremental test showed linear behavior until very high stress level corresponding to the hardening domain. The non-linear and tertiary creep was obtained during the last loading step, for 12 and 11 MPa respectively, which occurred in the hardening domain. The Acoustic Emission monitoring confirmed very close correlation between non-linear creep and AE cumulative number of events induced by microcracking. In order to precisely detect non-linear phenomena in incremental test, analysis with linear viscoelastic models was performed. Comparison between creep and relaxation test results showed that non-linear viscoelastic phenomena were more actively involved in creep tests. The outcomes of this research, bridging the material science and structural engineering, provide useful data to foster applications in new and existing structures, making the best use of the tensile viscoelastic potential of UHPFRC.

EPFL2011

Zum Tragverhalten von zugbeanspruchten Bauteilen aus Ultra-Hochleistungs-Faserbeton

Jörg Jungwirth

Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) can be considered, due to its high strength and improved durability, as one of the leading recent innovations in civil engineering. Material scientists have developed a very efficient, high-tech material by optimizing the concrete composition and improving the density of the cement matrix. Its structural behavior and in particular its relatively high ductility, despite the brittle behavior of the cement matrix, is influenced significantly by the crack-bridging behavior of the fibers. This behavior affects not only the tensile behavior, but also the global material behavior, i.e.: bending (tension by bending), compression (transversal tension, confinement), shear, punching, concentrated load introduction, etc. In the presented study, the tensile behavior of UHPFRC is analyzed as a basis for its structural behavior. The investigations extend from the 'micro-scale' behavior of the crack-bridging fibers in one crack to the 'macro-scale' behavior of tension members composed of UHPFRC. To obtain efficient tensile performance, the use of the UHPFRC in combination with reinforcing bars is expedient. The bond between the reinforcing bars and the UHPFRC is investigated, in order to analyze and understand the structural response of reinforced UHPFRC tension members. At each level of investigation, emphasis is placed on comprehension of the mechanical process. The influence of the fiber orientation, creep, shrinkage, and pre-stressing is discussed. Based on the studied mechanical processes, micro- and macro-scale modeling of the structural behavior is carried out. The cracking mechanisms of UHPFRC are examined in the research work. For the structural response of reinforced UHPFRC tension members, the influence of the double multi-cracking behavior is particularly relevant. First, multiple meso-cracks form controlled by the fibers and then multiple macro-cracks form controlled by the bond between the UHPFRC and the reinforcing steel. With a model based on these mechanisms, the structural response of tension members made from different types of UHPFRC can be described. Depending on the configuration of the tension member (i.e.: type of UHPFRC, reinforcement ratio, steel type, pre-stressing), five different failure regimes result for reinforced UHPFRC tension members. For practical applications the following conclusions can be drawn: Tension members composed of UHPFRC show very efficient structural behavior. The resistance of the UHPFRC supplements the steel resistance according to its strain-respective cracking state. The well distributed cracking of the UHPFRC leads to a considerable increase in the rigidity of the tension member (high tension stiffening effect). To obtain an efficient tension member with a large deformation capacity, a relatively high reinforcement ratio is necessary. Due to the associated continuous hardening behavior and significant elastic deformation, the use of high-strength reinforcing steel without a yielding plateau is favorable.

EPFL2006

Structural analysis of a composite bridge structure combining UHPFRC and prestressed concrete

Eugen Brühwiler, Cornelius Oesterlee, Hamid Sadouki

Reinforced concrete being the most applied construction material today performs very well in most applications but still lacks durability under severe environmental conditions. Especially existing structures built decades ago show degradation. Using Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) to improve durability is a promising option seen the extraordinary performance of this material when applied in a composite section. Restrained shrinkage of the overlay plays an important role in this type of application. A conceptual bridge cross section combining UHPFRC and reinforced concrete has been numerically analysed. Two main parameters, tensile strength and strain hardening capacity, were varied. The analysis indicates the importance of strain hardening of UHPFRC and the influence of its tensile strength on the structural response under restrained shrinkage (deformation controlled loading) and traffic loads (force controlled loading). The results validate the concept of using a UHPFRC layer to improve the structural durability of concrete constructions.